A number of proposed autostereoscopic (naked-eye) 3D displays or, more broadly, light field generator architectures utilize a variety of scanning, diffraction, space-multiplexing, steered illumination, and other techniques. One category, electro-holographic displays, relies principally on diffractive phenomena to shape and steer light. Examples of electro-holographic displays are described in: Jason Geng, Three-dimensional display technologies, Advances in Optics and Photonics, 5, 456-535 (2013). (see pp. 508-516) and Yijie Pan et al., A Review of Dynamic Holographic Three-Dimensional Display: Algorithms, Devices, and Systems, IEEE Transactions on Industrial Informatics, 12(4), 1599-1610 (August 2016). Electro-holographic light field generators hold the promise of projecting imagery with the ultimate in realism: curved optical wavefronts, which can genuinely replicate the real world. Such displays can theoretically provide nearly perfect characteristics of visual depth information, color rendering, optical resolution, and smooth transitions as viewers changes their location. So far, displays built on this technology have not achieved this theoretical level of performance.
One specific device category that provides controllable sub-holograms from which a light field can be constructed uses what is known as a surface acoustic wave (SAW) modulator. A SAW is generated in a piezoelectric substrate under radio frequency (RF) excitation. This creates a time-varying diffracting region that interact with input light in the waveguide. This causes at least some of the light to change from a guided mode within the waveguide to a leaky mode that exits the waveguide. This is described more fully, for example, in:
Onural et al., “New high-resolution display device for holographic three-dimensional video: principles and simulations,” Optical Engineering, vol. 33(3), pp. 835-44 (1994);
Matteo et al., Collinear Guided Wave to Leaky Wave Acoustooptic Interactions in Proton-Exchanged LiNbO3 Waveguides, IEEE Trans. on Ultrasonics, Ferroelectrics, and Frequency Control, 47(1), 16-28 (January 2000);
Smalley et al., Anisotropic leaky-mode modulator for holographic video displays, Nature, 498, 313-317 (20 Jun. 2013);
U.S. Pat. App. Publ. US 2014/0300695; Full Parallax Acousto-Optic/Electro-Optic Holographic Video Display;
Gneiting et al., Optimizations for Robust, High-Efficiency, Waveguide-Based Holographic Video, Industrial Informatics (INDIN), 2016 IEEE 14th International Conference on, (19-21 Jul. 2016);
Hinkov et al., Collinear Acoustooptical TM-TE Mode Conversion in Proton Exchanged Ti:LiNbO3 Waveguide Structures, J. Lightwave Tech., vol. 6(6), pp. 900-08 (1988);
McLaughlin et al., Optimized guided-to-leaky-mode device for graphics processing unit controlled frequency division of color, Appl. Opt., vol. 54(12), pp. 3732-36 (2015);
Qaderi et al., Leaky-mode waveguide modulators with high deflection angle for use in holographic video displays, Opt. Expr., vol. 24(18), pp. 20831-41 (2016); and
Savidis et al., Progress in fabrication of waveguide spatial light modulators via femtosecond laser micromachining, Proc. of SPIE Vol. 10115, (2017).
FIG. 1 shows an exemplary prior an SAW optical modulator 100. It can be used to deflect light of the same or different colors/wavelengths 101a, 101b, 101c from guided modes by different angles simultaneously, or serially, in time.
The modulator 100 comprises a substrate 120 in which or on which an optical waveguide 102 has been formed. The input light 101a, 101b, and/or 101c at one or more wavelengths (λ1, λ2, λ3) enters waveguide 102. An in-coupling device 106 is used to couple the input light 101 carried in an optical fiber, for example, into the waveguide 102. Examples of in-coupling devices 106 include in-coupling prisms, gratings, or simply butt-coupling between an optical fiber or light in free-space and the waveguide 102. The input light 101 is launched into a guided mode upon entry into the waveguide 102. Commonly, the TE (transverse electric) mode is guided.
In such a SAW modulator 100, the waveguide 102, e.g., slab waveguide, is typically created in a lithium niobate substrate 120 by proton-exchange. Transducers (e.g., interdigital transducers (IDTs)) 110 are written on an aluminum side of the substrate 120. The transducers 110 induce surface acoustic waves (SAWs) 140 in the substrate 120 that propagate along the waveguide 102. Such transducers 110 are often driven electrically, e.g. using a 300-500 MHz radio frequency (RF) drive signal 130.
The light interacts with the surface acoustic wave 140. The result of this interaction between the SAW 140 and the light in the waveguide 102 is that a portion of the guided light is polarization-rotated, out of the guided mode and into a leaky mode having the transverse magnetic (TM) polarization. The light then exits the waveguide 102 as leaky-mode or diffracted light 162 and enters substrate 120 at angle φ, measured from grazing 77. At some point this diffracted light 162 exits the substrate 120 at an exit face, which is possibly through the substrate's distal face 168 or end face 170 (as shown) as exit light 150 at an exit angle of θ. The range of possible exit angles θ comprises the angular extent, or exit angle fan, of the exit light 150.
Practical electronic constraints and materials properties often limit the resulting angular deflection of SAW devices. Qaderi (2016) reports that a total output angle of approximately 20° can be achieved, significantly lower than the field of view of contemporary 2D displays that approach 180°. Existing electro-holographic 3D displays using SAW devices have attempted to increase the exit angle fan of the diffracted output light 150 in various ways such as by optimizing various modulator parameters to increase the useful bandwidth of the RF driver such as waveguide depth and IDT design (in published systems, the output angle is a function of IDT drive frequency), by using edge-emitting modulators having “right-angle” edges, by doubling the angle fan via waveguides on both sides of the wafer, and/or by demagnification (i.e. using a large lens to demagnify an area of numerous modulators to provide a smaller visible display area having larger field of view). But it does not appear that any of these are adequate to achieve an angle fan as high as 90° in any sort of flat form-factor.